1. Field of the Invention
2. Description of the Prior Art
The invention relates to a method as well as a device for producing wood chips. For this, there are basically two systems, which differ fundamentally from each other and which, accordingly, also produce different products.
The first system relates to so-called direct chippers. In it, uncut round timber is fed into a chute. Arranged parallel to the axis along which the wood is fed is a knife ring, which is equipped along its circumference with a large number of knives. The cutting edges of the knives run parallel to the knife ring. The knife ring can move in such a way that it plunges into the body of the round timber and is able to chip it. See, for example,
DE 2,947,199 C1.
Another design of direct chippers, which belongs to the first system mentioned, has a disk that is mounted so that it can pivot. The disk carries chipping knives on one of its side faces. The round timber or tree stumps are fed to this working face in a direction parallel to the fibers. Because the circumferential speeds of the individual face elements of the working face differ in magnitude, depending on whether the face element lies radially inward or radially outward, the chipping result also differs and this is detrimental to the quality.
The machines mentioned, which belong to the first system, serve, above all, to produce so-called oriented strand (OS) chips. The fibers of these chips are oriented along the lengthwise direction of the chip. The chip is extremely thin. It serves to produce boards of the same name, so-called oriented strand boards (OSB). These are regarded as being of high quality. They have great strength, so that they can be used as construction elements for the construction of prefabricated houses, for example.
The second system mentioned uses so-called knife ring chippers. These consist of a ring or collar that is made up of knives, whose cutting edges run, at least largely, parallel to the ring axis. The knife ring encloses a working chamber in which the chippings are introduced into a radially inner region through a filling shaft. A rotor spins the chippings radially outward against the inner face of the knife ring and thus against the cutting edges of the knives, where chipping takes place.
The intermediate product that is fed to the chipper consists of appreciably smaller particles, namely, chippings, that were produced beforehand in a chopping process. In quite generalized terms, they have, at least to an approximation, the size of a match box.
The chips that can be produced by a knife ring chipper differ quite fundamentally from the OS chips produced by the first-mentioned category of machines, which operate according to the first-mentioned system. They are essentially pin-shaped and are reminiscent of thin matches. These chips can be processed only into chipboards or particle boards. However, particle boards are of only limited strength and, in particular, they have no special flexural strength. Thus, they cannot be used as construction elements, but instead find application in the furniture industry.
The advantages and drawbacks of machines of the first-mentioned system—direct chippers for producing OS chips—and of the second-mentioned system with the preceding stage for producing chippings and the final result of chips for particle boards can be summarized as follows:
Direct chippers require, in general, fresh wood, that is, tree trunks of, for example, one meter. Although they are capable of producing high-quality OS chips, they have comparatively large dimensions on account of the large dimensions of the intermediate product. Accordingly, it is extremely expensive to purchase them. The investment costs lie between one and three million DM. Accordingly, they also have relatively little flexibility in use.
Chippers, by contrast, are smaller, more flexible, and more advantageous in cost to purchase. However, they are not capable of producing OSB chips, but only the lower-quality matchlike chips for particle boards.